Add mc_is_infrastructure flag and feeder/admin CRUD for MeshCore infra nodes

[pskillen]

Summary

MeshCore infrastructure listing should not return every observed MC node. Add an explicit operator-controlled flag on ObservedNode and tighten the infrastructure endpoint.

UI tracking: meshflow-ui#307

Background

GET /api/nodes/observed-nodes/infrastructure/?protocol=meshcore currently returns all protocol=MESHCORE rows in the time window. Meshtastic infra correctly uses meshtastic_role ∈ INFRASTRUCTURE_ROLES. MeshCore has no equivalent role model; filtering by repeater/type alone is too loose.

Design

ObservedNode.mc_is_infrastructure

• BooleanField, default=False
• Meaningful only for protocol=MESHCORE; ignore or reject writes on Meshtastic rows
• Does not replace MT infrastructure semantics

Infrastructure list

• protocol=meshtastic (default): unchanged (protocol=MESHTASTIC + infrastructure roles)
• protocol=meshcore: protocol=MESHCORE and mc_is_infrastructure=True

Write API

• Expose on observed-node PATCH and/or dedicated action (e.g. POST …/observed-nodes/{id}/set-mc-infrastructure/)
• Permissions: Django feeder group or is_staff only; deny for plain authenticated users
• Document in docs/permissions/README.md and openapi.yaml

Tasks

• Migration + model field
• Serializer(s) + validation (MC only)
• Permission class / object checks
• Update infrastructure action filter for meshcore
• Unit tests (permissions + queryset)
• Optional: admin list filter

Acceptance criteria

1. Only flagged MC nodes appear in meshcore infrastructure API results.
2. Non-feeder non-admin users receive 403 on flag writes.
3. MT infrastructure list unchanged.